1. Field of the Invention
The present invention relates to a Faraday rotator, and more particularly, to a Faraday rotator for use in a variable optical attenuator etc.
2. Description of the Related Art
Optical communication technology is a key to the formation of the basis of multimedia communications, and there has been a demand for further advanced services with wider coverage. Also, with the recent explosive spread of the Internet, advancement of optical networks capable of large-capacity transmission is demanded. Under the circumstances, communication techniques such as WDM (Wavelength Division Multiplexing) communication in which optical signals with different wavelengths are multiplexed for transmission are attracting attention and are under development.
Meanwhile, due to the tendency toward larger capacity of optical networks, optical devices are also rapidly diversifying and are required to have high functionality. Typical examples of optical devices include variable optical attenuator, optical shutter, variable optical equalizer, etc.
The variable optical attenuator (VOA) is a device for variably controlling the level of optical signal in order to properly set a level diagram despite fluctuations in optical level.
The optical shutter is a device for preventing high optical power amplified in the process of optical communication from damaging the human body in case of disengagement of a connector or the like. When disengagement of the connector is detected, the optical shutter shuts down the transmitting optical output.
The variable optical equalizer is a device for equalizing gain by controlling the sum of gains of an EDFA (Erbium-Doped Fiber Amplifier) to a fixed value, to thereby flatten the wavelength-dependent gain characteristic within the signal band. EDFA is an optical amplifier (having a wide amplifiable wavelength range and a nonnegligible wavelength characteristic) using, as a medium for amplification, an optical fiber doped with erbium (Er3+) and is widely used in repeaters etc. for WDM transmission.
A Faraday rotator is used as a principal component part of optical devices such as a variable optical attenuator, optical shutter, variable optical equalizer and the like. The Faraday rotator uses a magneto-optical crystal having the property of causing rotation of the plane of polarization of transmitting light by magnetic field (this rotation is called Faraday rotation and the angle of rotation is called Faraday rotation angle) and substantially controls the transmittance of light by means of the Faraday rotation angle.
Usually, the Faraday rotator uses, as its magneto-optical crystal, a yttrium iron garnet (YIG) crystal (hereinafter referred to as garnet single crystal) or the like. Recently, a film of bismuth-substituted rare-earth iron garnet single crystal formed by liquid-phase epitaxy has also come to be used. The bismuth-substituted rare-earth iron garnet single crystal film is superior to the garnet single crystal in that it has a large Faraday rotation coefficient.
However, both of the garnet single crystal and the bismuth-substituted rare-earth iron garnet single crystal film have a common disadvantage that the Faraday rotation angle is highly temperature-dependent (The Faraday rotation angle fluctuates in response to changes in ambient environmental temperature).
Thus, the Faraday rotation angle of a Faraday rotator shows a temperature characteristic, which is given by the sum of the temperature dependency of the Faraday rotation angle itself and the temperature dependency of magnetic anisotropy.
The magneto-optical crystal has an axis along which the crystal can be easily magnetized (easy magnetization axis) and an axis along which it is hard to magnetize the crystal (hard magnetization axis), depending on its crystal axis. The phenomenon showing different magnetic properties in different directions is called magnetic anisotropy. The magnitude of magnetic anisotropy (magnitude of energy directing magnetization along the hard magnetization axis) increases with decreasing temperature (Faraday rotation is less liable to occur) and decreases with increasing temperature (Faraday rotation is more liable to occur).
Because of the temperature characteristic of the Faraday rotator, the temperature stability of the variable optical attenuator, optical shutter and variable optical equalizer is impaired. Accordingly, the temperature characteristic of the Faraday rotator needs to be improved, in order to enhance the operation stability of such optical devices against changes in environmental temperature.
To improve the temperature characteristic of the Faraday rotator, a method has conventionally been employed in which the magneto-optical crystal is disposed in a manner such that the sign of the temperature coefficient of the crystal itself is opposite to that of the temperature coefficient of the magnetic anisotropy, to offset (cancel out) the temperature dependency of the Faraday rotation angle itself by the temperature dependency of the magnetic anisotropy and thereby suppress the temperature characteristic of the Faraday rotator.
FIG. 32 illustrates Faraday rotation angles, wherein the temperature-dependent Faraday rotation angle characteristic is canceled out by the temperature-dependent magnetic anisotropy characteristic according to the conventional method. The vertical axis indicates Faraday rotation angle and the horizontal axis indicates current (driving current passed through the coil wound around the electromagnet constituting the Faraday rotator). FIG. 32 shows Faraday rotation angles measured at temperatures of 0° C. and 65° C.
In a range H within which the temperature characteristics are canceled out, the Faraday rotator shows a small temperature characteristic and thus the Faraday rotation angle is not dependent on temperature (Namely, in the range H, the Faraday rotation angle is almost the same at both 0° C. and 65° C.). Thus, according to the conventional technique, the temperature-dependent Faraday rotation angle characteristic is canceled out by the temperature-dependent magnetic anisotropy characteristic, to thereby suppress and improve the temperature characteristic of the Faraday rotator.
As seen from FIG. 32, with the conventional technique, the Faraday rotator shows a small temperature characteristic only in the vicinity of a specific Faraday rotation angle falling within the range H (This is because, while the temperature coefficient of the Faraday rotation angle of the magneto-optical crystal is independent of the Faraday rotation angle and is almost constant, the temperature coefficient of the magnetic anisotropy exerts an influence only within a narrow range of orientation restricted to the vicinity of the specific Faraday rotation angle).
Consequently, the conventional technique is associated with a problem that, although the temperature-dependent Faraday rotation angle characteristic can be suppressed within the narrow range corresponding to the range H, the temperature characteristic cannot be suppressed over a wide range of Faraday rotation angle beyond the range H.
Temperature-dependent fluctuation (fluctuation of the Faraday rotation angle dependent on temperature) of a conventional Faraday rotator will be now described with reference to specific numerical values. FIG. 33 illustrates temperature-dependent Faraday rotation angle characteristics of the conventional Faraday rotator, wherein the vertical axis indicates Faraday rotation angle (deg.) and the horizontal axis indicates current (mA). FIG. 33 shows Faraday rotation angles observed at environmental temperatures of 0° C. (solid line), 25° C. (thick solid line) and 65° C. (dotted line), respectively.
FIG. 34 shows temperature-dependent fluctuation of the Faraday rotation angle of the conventional Faraday rotator. FIG. 34 illustrates a differential angle of the Faraday rotation angles shown in FIG. 33 (difference between maximum and minimum values among the three rotation angles at 0° C., 25° C. and 65° C.) within a range of current from 20 mA to 100 mA. The vertical axis indicates temperature-dependent rotation angle fluctuation (deg.), which is the differential angle, and the horizontal axis indicates current (mA).
In cases where the Faraday rotator is applied to a variable optical attenuator or variable optical equalizer, it is necessary that the temperature characteristic should be of a satisfactory level at and below a Faraday rotation angle of about 40 degrees. As shown in FIG. 33, however, the Faraday rotation angle exhibits a temperature characteristic even at a Faraday rotation angle of about 40 degrees within the temperature range of 0° C. to 65° C. (Namely, at a Faraday rotation angle around 40 degrees, the temperature characteristic curves in the graph are separated from one another and do not coincide to form a single line).
To confirm this with reference to FIG. 34, it is clearly shown that the Faraday rotation angle undergoes a temperature-dependent fluctuation of about 1.5 degrees within the range of the driving current from 20 mA to 100 mA, the driving current being supplied to the electromagnet constituting the Faraday rotator. Such large temperature-dependent fluctuation is not allowable for optical devices including a Faraday rotator as a component part.
The following describes problems with various optical devices using the conventional Faraday rotator. FIG. 35 illustrates temperature-dependent fluctuation of a variable optical attenuator using the conventional Faraday rotator. The vertical axis indicates attenuation deviation (dB) and the horizontal axis indicates current value (mA), the temperature range being from 0° C. to 65° C.
As shown in FIG. 35, a fluctuation of approximately 3 dB is observed within the range of the driving current value from 0 mA to 100 mA. A temperature-dependent fluctuation of 3 dB is not allowable in the case of setting the optical level for optical communications.
FIG. 36 illustrates temperature characteristics of an optical shutter using the conventional Faraday rotator. The vertical axis indicates amount of attenuation (dB) and the horizontal axis indicates current (mA). As shown in FIG. 36, the attenuation-to-current characteristics show significant temperature characteristics in a region where the amount of attenuation is large.
For example, where the driving current is 50 mA, an attenuation of 45 dB is obtained at 65° C., but the attenuation decreases to 28 dB at a temperature of 25° C. and drops further to 25 dB at a temperature of 0° C.
Thus, with the optical shutter using the conventional Faraday rotator, the amount of attenuation at the time of shut-down greatly varies depending on the environmental temperature, even if the current value is the same. Conventionally, therefore, a feedback control circuit must be separately provided for supplying the optical shutter with a suitable current value corresponding to the detected environmental temperature to set the amount of attenuation for the shut-down, but this leads to an increase in the cost of the optical shutter.
FIG. 37 illustrates temperature characteristics and temperature-dependent equalization deviation of a variable optical equalizer using the conventional Faraday rotator. The left-hand vertical axis indicates amount of attenuation (dB), the right-hand vertical axis indicates temperature-dependent equalization deviation (dB), and the horizontal axis indicates wavelength (nm).
From the data showing the temperature-dependent equalization deviation (gain equalization level dependent on temperature fluctuations), it is apparent that the temperature-dependent equalization deviation of the variable optical equalizer using the conventional Faraday rotator is as high as 0.7 dB in a wavelength range of input light from 1520 nm to 1560 nm.
Let us consider an optical communication system such as a WDM system having n stages (n is a positive integer) of repeaters with random characteristics. If the equalization deviation caused in one stage is 0.7 dB, an equalization deviation of n1/2×0.7 dB is caused for n stages (e.g., about 2 dB for 10 stages, and 7 dB for 100 stages). It is difficult to apply variable optical equalizers having such characteristics to WDM systems.